1. Field of the Invention
The present invention relates to an optical encoder that detects a position in a linear direction or a rotational direction.
2. Description of the Related Art
Conventionally, an encoder is used to detect position in a machine tool, a factory automation machine, and the like. A position detection method of encoder is divided roughly into an incremental method for measuring a relative moving distance, which is a relative value of a position, and an absolute method for detecting an absolute value of a position. Although the configuration of the incremental method is simple, there are problems that position information is lost when power is turned off and errors are accumulated by exogenous noise. On the other hand, an encoder of the absolute method is generally highly accurate without errors being accumulated, and has an advantage that it is not necessary to move to a home position even when power is turned off.
For example, an encoder of the absolute method, which can output an absolute value as well as a relative value related to a position in the measuring direction, has a configuration as shown in FIG. 19 (EP2093543). According to this example, a large amount of information can be obtained from a small number of scale tracks by forming a plurality of modulations having different pitches in a scale pattern on one track in order to output a relative value.
When performing analysis on the basis of the prior example, it is found that a light intensity distribution, which is reflected from a scale including a plurality of modulation pitches, includes unnecessary spatial frequency components that may cause errors in position detection. Specifically, in the prior example, although the light intensity distribution corresponds to a sum of a modulation component A corresponding to a fine pitch pattern and a modulation component B corresponding to a coarse pitch pattern, the light intensity distribution includes unnecessary spatial frequency components (A+B, A−B, and further, components due to unnecessary diffracted light) that may cause errors in position detection. By the effect of the above, an error from an ideal sine wave occurs. As a result, there is a problem that a position detection error occurs when converting the sine wave into a position signal by an arctangent calculation.